The disclosure relates to a semiconductor memory device, and more particularly, to a nonvolatile memory device and a method of reading the same.
A semiconductor memory device is classified into a volatile semiconductor memory device and a nonvolatile semiconductor memory device.
A volatile semiconductor memory device has a high read speed and a high write speed. However, it loses its stored data when its power is interrupted. A nonvolatile semiconductor memory device retains its stored data even when its power is interrupted. Thus, the nonvolatile semiconductor memory device is used to remember contents that have to be preserved regardless of whether power is supplied or not. Examples of a nonvolatile semiconductor memory device include an MROM (mask read only memory), a PROM (programmable ROM), an EPROM (erasable programmable ROM), an EEPROM (electrically EPROM), etc.
Generally, because an erase operation and a write operation cannot be performed autonomously in an MROM, a PROM and an EPROM, it is not easy for general users to renew memory contents. Since an EEPROM is capable of electrically performing an erase operation and a write operation, an application of the EEPROM extends to system programming that requires continuous renewal or an auxiliary memory device. In particular, since a flash EEPROM has a high integration compared with an existing EEPROM, it is of great advantage to an application to have a large capacity auxiliary memory device. A NAND type flash EEPROM (hereinafter it is referred to as ‘NAND-type flash memory’) among flash EEPROMs has a high integration compared with other flash EEPROMs.
In a flash memory device, a data state capable of being stored in each memory cell is determined depending on the number of bits being stored in each memory cell. A memory cell storing 1-bit data in one memory cell is called a single bit cell or a SLC (single-level cell). A memory cell storing multi-bit data in one memory cell is called a multi-bit cell, a MLC (multi-level cell) or a multi-state cell. As a high integration requirement for a memory device increases, a study of a multi-level flash memory storing multi-bit data in one memory cell is vigorously proceeding.